[reg] Fix shadow reg update during storage error
[reg] Fix shadow reg update during storage error

- Fixes #7764
- Do not allow register upates once storage errors occur.
  This effectively makes the storage error persistent.

Signed-off-by: Timothy Chen <d7b2ae30d8c0f215facd721941273067fd1d14a2@google.com>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan